1. Field of the Invention
The present invention generally relates to a method and apparatus for sensing, receiving, and processing light signals and, more particularly, to a method and apparatus for accurately resolving relative times-of-arrival of light pulses.
2. Description of the Related Art
In general, three-dimensional imaging systems employing active sources, such as laser detection and ranging (LADAR) systems, suffer from one primary problem: sensors designed to obtain two-dimensional amplitude images are not adept at rendering an image in three-dimensions. While there have been many attempts at adopting such two-dimensional sensors to three-dimensional imaging, such systems have always been found to be lacking, particularly in range resolution and sensitivity.
For example, one type of known three-dimensional imaging approach uses very high pixel sampling rates in various forms to determine time of flight for the laser pulse to travel from the laser to a target and on to a detector. The time of flight of an illuminating pulse is very difficult to measure since one nanosecond of time resolution is required to achieve one foot of depth resolution. As such, these systems typically employ high-speed counting and high-speed clocking circuits for operation. In cases where a depth resolution of inches is necessary (i.e., sub-nanosecond time differences must be resolved), the required operating speed of these counting and clocking circuits is difficult to achieve. Other known systems measure phase shifts between the illuminating signal and the signal returned from the target. These systems are susceptible to noise and provide inadequate sensitivity when the signal reflected from the target is very weak.
Therefore, there exists a need in the art for a method and apparatus for accurately resolving sub-nanosecond differences between times-of-arrival of light pulses.
The present invention is a device for resolving relative times-of-arrival of a plurality of light pulses comprising a plurality of drift-field detectors. Each drift-field detector comprises a light sensor and a semiconductor drift region. Each light sensor generates an electrical charge from at least one of the plurality of light pulses. Each semiconductor drift region receives the electrical charge from its respective light sensor and, pursuant to an electric field therein, produces a spatial charge distribution. The spatial charge distribution for each of the semiconductor drift regions is stored in an analog storage device associated therewith. In one embodiment of the invention, the analog storage devices comprise charge-coupled device (CCD) registers. The relative positions of the charge distributions in the semiconductor drift regions can be used to calculate the relative times-of-arrival of the light pulses. The present invention can be used in three-dimensional imaging applications, where the relative times-of-arrival of reflected light pulses are used to calculate the depth of the scene.